QImageReader::setScaledSize() allows to use fast downscaling
for jpeg images (by setting jpeg_decompress_struct::scale_denom).
QImageReader::setQuality(0) sets dct_method to JDCT_IFAST and
do_fancy_upsampling to FALSE, although these parameters don't
really influence on decoding speed, also as do_block_smoothing
which is not used by Qt.
QImageReader::setAutoTransform() allows to apply the rotation
from EXIF data. It was introduced in Qt 5.5, so bump the
required Qt version.